The present application claims priority to Japanese Application(s) No(s). P2000-308301 filed Oct. 6, 2000, which application(s) is/are incorporated herein by reference to the extent permitted by law.
1. Field of the Invention
This invention relates to a non-aqueous electrolyte secondary cell employing a so-called olivinic lithium phosphorus oxide as a cathode active material.
2. Description of Related Art
Nowadays, in keeping up with the recent marked progress in the electronic equipment, researches into re-chargeable secondary cells, as power sources usable conveniently and economically for prolonged time, are underway. Representative of the secondary cells are lead accumulators, alkali accumulators and non-aqueous electrolyte secondary cells.
Of the above secondary cells, lithium ion secondary cells, as non-aqueous electrolyte secondary cells, have such merits as high output and high energy density.
The lithium ion secondary cells are made up of a cathode and an anode, including active materials capable of reversibly doping/undoping lithium ions, and a non-aqueous electrolyte. The charging reaction of the lithium ion secondary cell proceeds as lithium ions are deintercalated into an electrolyte solution at the cathode and are intercalated into the anode active material. In discharging, reaction opposite to that of the charging reaction proceeds, such that lithium ions are interecalated at the cathode. That is, charging/discharging is repeated as the reaction of entrance/exiting of lithium ions from the cathode into the anode active material and from the anode active material occurs repeatedly.
As the cathode active material of the lithium ion secondary cell, LiCoO2, LiNiO2 or LiMn2O4 is used because these materials have a high energy density and a high voltage. However, these cathode active materials, containing metal-elements of low Clark number in their composition, suffer from high cost and supply instability. Moreover, these cathode active materials are higher in toxicity and affect the environment significantly. So, there is presented a demand for a novel substitution material usable as a cathode active material.
Proposals have been made for use of LiFePO4, having an olivinic structure, as a cathode active material for a lithium ion secondary cell. LiFePO4 has a volumetric density as high as 3.6 g/cm3 and generates a high potential of 3.4V, with its theoretical capacity also being as high as 170 mAh/g. Additionally, LiFePO4 contains an electrochemically undopable Li at a rate of one atom per Fe atom, in its initial state, and hence is promising as a cathode active material for a lithium ion secondary cell. Moreover, LiFePO4 includes iron, as an inexpensive material plentiful in supply, in its composition, and hence is less costly than any of the aforementioned materials, that is LiCoO2, LiNiO2 or LiMn2O4. Additionally, LiFePO4 is low in toxicity and hence is less aggressive to environment.
Although the olivinic lithium phosphorus oxides have the advantages as described above, there are difficulties that need to be surmounted if it is attempted to use the material for a cell.
For example, the oliviniic lithium phosphorus oxide is only poor in electronic conductivity, so that, if it is used as cathode active material, it is difficult to realize sufficient load characteristics.
For improving the load characteristics, it may be envisaged to reduce the thickness of the layer of the cathode active material. However, in this case, the proportion of the collector is relatively increased, so that the problem of the lowering of the capacity is raised.
It is therefore an object of the present invention to provide a non-aqueous electrolyte secondary cell in which, when a compound having an olivinic structure is used as a cathode active material, the load characteristics and the cell capacity can be compromised relative to each other to realize a non-aqueous electrolyte secondary cell having superior cell characteristics.
The present invention provides a non-aqueous electrolyte secondary cell including a cathode having a layer of a cathode active material containing a compound represented by the general formula LixFe1-yMyPO4, where M is at least one selected from the group consisting of Mn, Cr, Co, Cu, Ni, V, Mo, Ti, Zn, Al, Ga, Mg, B and Nb, with 0.05xe2x89xa6xxe2x89xa61.2 and 0xe2x89xa6yxe2x89xa60.8, an anode having a layer of a cathode active material containing an anode active material, and a non-aqueous electrolyte, wherein the layer of the cathode active material has a film thickness in a range from 25 to 110 xcexcm.
Since the olivinic lithium phosphorus oxide is poor in electronic conductivity, it is difficult to realize sufficient load characteristics with the use of this oxide as a cathode active material. If load characteristics are taken into consideration, it is meritorious to reduce the film thickness of the layer of the cathode active material.
However, if the layer of the cathode active material is too thick, the proportion of the current collector in the inner cell capacity is relatively increased to lower the cell capacity.
In this perspective, the film thickness of the layer of the cathode active material is set to 25 to 110 xcexcm.
With the thickness of the layer of the cathode active material in the above range, practically sufficient load characteristics can be achieved, whilst the cell capacity may be of a practically satisfactory level.
In the non-aqueous electrolyte secondary cell of the present invention, in which an olivinic lithium phosphorus oxide is used as a cathode active material, and the thickness of the layer of the cathode active material is optimized, the load characteristics can be compromised relative to the cell capacity to realize a non-aqueous electrolyte secondary cell having superior cell performance.